This invention relates to run length decoding apparatus which decodes a series of encoded data into original data.
Encoding of run lengths is used for compressing various data such as facsimile signals, and other data.
For example, a modified Huffman encoding system (MH encoding) utilized as the international standard facsimile system is a typical one.
In an ordinary facsimile system, it takes about 30 seconds for transmitting a manuscript of A4 size (300 mm.times.210 mm) so that even when an interval of the same order is necessary for decoding it is not necessary to decode at a higher speed. In a case where data compressed by run length encoding are stored in a memory device and the stored data are decoded when it is desired to display the decoded data on a television display device, the time necessary for decoding must be short.
A typical example of decoding compressed data of a A4 maniscript in 0.1 to 0.2 seconds will now be considered. Since the number of the picture elements per one scanning line is 1728 and since the number of the scanning lines per page is about 2300 the total number of the picture elements amounts to about 4 mega-bits. Supposing a data compression ratio of 10, the number of the compressed data would be 0.4 mega-bits. The process of decoding comprises a first step in which the compressed data are read out from the memory device and decoded, and then the decoded data are separated into data respectively representing individual white or black run lengths, and a second step in which decoded data corresponding to white and black of respective picture elements are generated from data representing the run lengths. Denoting the frequency of an operating clock pulse by f megahertz and supposing that one datum is decoded by one clock pulse, the time T1 required for the first step is 0.4/f seconds. Where decoded data of one picture element is produced by one clock pulse, the time T2 required for the second step is 4/f seconds. For example, where the clock frequency f is equal to 10 megahertz, T1=0.04 sec., and T2=0.4 sec., so that afore-mentioned target can not be reached. To attain the target value of 0.1 sec., it is necessary to increase the clock frequency to 40 megahertz, which requires a special, expensive and ultra high speed circuit element.